In addition to conducting signalling, voice and data communications via the tip and ring leads of a multi-lead telephone link (containing tip, ring, sleeve and ground leads), the sleeve lead may be used for controlling the operation of a unit, such as a no-test trunk (NTT). For example, U.S. Pat. No. 4,841,560, to A. Chan et al, entitled "Direct Access Test Unit for Central Office," issued June 20, 1989, assigned to the assignee of the present application and the disclosure of which is herein incorporated, describes a direct access test unit (hereinafter referred to as a DATU) that is installed within the equipment frame of a central office for the purpose of enabling a craftsperson to conduct tests of subscriber lines without the intervention of central office personnel. A primary function of the DATU is to control the operation of a no-test trunk within the central office.
For this purpose, the DATU may place the sleeve lead into one of a plurality (five) of predefined current flow states, each of which may be associated with a specific operational condition of the no-test trunk. Moreover, transitions between two different sleeve states may be used to invoke preselected no-test trunk operations. In such a system, regardless of which sleeve current flow state is associated with a particular control function, it is important that continuity of the sleeve current flow state, as monitored by the no-test trunk, be maintained, so that there is no disruption of intended no-test trunk operation.